Synthesis of melamine polyphosphate with low residual melamine

Abstract

The present invention relates to a method for producing a melamine depleted melamine polyphosphate which comprises heating a crude melamine polyphosphate for a hold time of at least one hour to a hold temperature of above 290° C.; to a melamine depleted melamine polyphosphate which contains a residual melamine and where the residual melamine concentration is below 0.2 wt %; and to a polymer composition comprising the melamine depleted melamine polyphosphate.

1 . A method for producing a melamine depleted melamine polyphosphate which comprises heating a crude melamine polyphosphate for a hold time of at least one hour to a hold temperature of above 290° C. 
     
     
         2 . The method according to  claim 1  where the crude melamine polyphosphate has a residual melamine concentration of at least 0.2 wt % melamine. 
     
     
         3 . The method according to  claim 1  where the melamine depleted melamine polyphosphate has a residual melamine concentration of below 0.2 wt %, 0.1 wt % or 0.05 wt %. 
     
     
         4 . The method according to  claim 1  where the hold temperature is at least 295, 300, 305, 310 or 315° C. 
     
     
         5 . The method according to  claim 1  where the hold time is at least 1, 2, 3, 4 or 5 hours. 
     
     
         6 . The method according to  claim 1  where during the hold time the crude melamine polyphosphate is purged with an inert gas or subjected to a vacuum. 
     
     
         7 . The method according to  claim 1  where the crude melamine polyphosphate is produced by polymerizing a melamine phosphate for a polymerization time of at least 0.5 hour at a polymerization temperature in the range of 200 to 290° C. 
     
     
         8 . The method according to any of  claim 7  where the melamine phosphate contains melamine and phosphoric acid in a molar ratio of 1:1 to 2:1, preferably of 1.2:1 to 1.5:1. 
     
     
         9 . The method according to  claim 7  where the melamine phosphate is prepared by heating melamine, phosphoric acid and water above 70° C. 
     
     
         10 . The method according to  claim 7  where the melamine phosphate is prepared by adding phosphoric acid and water to a powder of melamine. 
     
     
         11 . The method according to  claim 1  where the melamine depleted melamine polyphosphate contains 1 to 30 wt % melam polyphosphate. 
     
     
         12 . A melamine depleted melamine polyphosphate which contains a residual melamine and where the residual melamine concentration is below 0.2 wt %, 0.1 wt % or 0.05 wt %. 
     
     
         13 . The melamine depleted melamine polyphosphate according to  claim 12  which contains 1 to 30 wt % melam polyphosphate. 
     
     
         14 . The melamine depleted melamine polyphosphate according to  claim 12  which contains 0.01 to 5.0 wt % melem polyphosphate. 
     
     
         15 . The melamine depleted melamine polyphosphate according to  claim 12 , where the melamine depleted melamine polyphosphate is obtainable by the method as defined in  claim 1  comprising heating the crude melamine polyphosphate for the hold time of at least one hour to the hold temperature of above 290° C. 
     
     
         16 . A polymer composition comprising the melamine depleted melamine polyphosphate as defined in  claim 13 .